The present invention relates to a method of crystal growth of a gallium nitride layer on gallium arsenide, and more particularly to a method of crystal growth of a gallium nitride layer with an extremely flat surface on gallium arsenide.
Gallium nitride, for application having a large energy band gap, offers a great deal of promise to various light emitting devices such as light emission diodes and semiconductor laser devices. Generally, gallium nitride is formed by crystal growth on a single crystal substrate.
Sapphire substrate has been most frequently used for crystal growth of a gallium nitride layer thereon. It was reported by S. Nakamura in J. Vac. Sci. Technol. Vol. 13, No. 3, p. 705, May/June 1995 that a light emission diode has a multilayer structure including a gallium nitride layer over a sapphire substrate. FIG. 1 is illustrative of this conventional light emitting diode. The conventional light emitting diode is formed over a (0001) surface of a sapphire substrate 701. An n-GaN low temperature growth buffer layer 702 is grown at a low temperature of 510.degree. C. over the sapphire substrate 701. The n-GaN low temperature growth buffer layer 702 has a thickness of 30 nanometers. An n-GaN layer 703 doped with silicon is formed at a temperature of 1020.degree. C. over the n-GaN low temperature growth buffer layer 702. The n-GaN layer 703 has a thickness of 4 micrometers. An n-Al.sub.0.15 Ga.sub.0.85 N layer 704 doped with silicon is formed at a temperature of 1020.degree. C. over the n-GaN layer 703. The n-Al.sub.0.15 Ga.sub.0.85 N layer 704 has a thickness of 0.15 micrometers. An In.sub.0.06 Ga.sub.0.94 N layer 705 doped with silicon and zinc is formed at a temperature of 800.degree. C. over the n-Al.sub.0.15 Ga.sub.0.85 N layer 704. The In.sub.0.06 Ga.sub.0.94 N layer 705 has a thickness of 100 nanometers. A p-Al.sub.0.15 Ga.sub.0.85 N layer 706 doped with magnesium is formed at a temperature of 1020.degree. C. over the In.sub.0.06 Ga.sub.0.94 N layer 705. The p-Al.sub.0.15 Ga.sub.0.85 N layer 706 has a thickness of 0.15 micrometers. A p-GaN layer 707 doped with magnesium is formed at a temperature of 1020.degree. C. over the p-Al.sub.0.15 Ga.sub.0.85 N layer 706. The p-GaN layer 707 has a thickness of 0.5 micrometers. A p-electrode 708 is formed on the p-GaN layer 707. The p-electrode 708 comprises double layers of nickel and gold. An n-electrode 709 is formed on a surface of the n-GaN layer 703. The n-electrode 709 comprises double layers of titanium and aluminum.
A GaN layer grown on the sapphire substrate has a better planarity and higher crystal quality than a GaN layer grown on other substrates. However, a sapphire substrate is not conductive and has a difficulty for cleavage thereof. Further, there are not available prior art process techniques for when GaAs substrate and InP substrate are used. When a sapphire substrate is used, it is necessary that both the p-electrode and n-electrode are formed on surfaces of the device, for which reason the processes for forming the electrodes are complicated.
On a other hand, if the GaAs substrate is used for formation of the GaN layer, then the GaAs substrate is electrically conductive and it is possible to make a cleavage thereof. Further, it is possible to employ prior art process techniques when GaAs substrate and InP substrate are used. It is disclosed by Qintin Guo and Hiroshi Ogawa, in Appl. Phys. Lett. 66, 715 (1995) that a GaN layer may be grown on a (111) surface of a GaAs substrate. It is also disclosed by in Appl. Phys. Lett. 78, 1842 (1995) that a GaN layer may be grown on a (100) surface of a GaAs substrate. If, however, an initial nitrization of a GaAs surface is not sufficient, then it is difficult to obtain high crystal quality of a GaN layer over a GaAs substrate. The GaN layer over a GaAs substrate is inferior in planarity than the GaN layer over a sapphire substrate. This problem is disclosed in Appl. Phys. Lett. 59, 1058 (1991), and Japan J. Appl. Phys. Lett. 30, L1665 (1991), and J. Vac. Sci. Technol. B9, 1924 (1991).
There is a method of initial nitrization wherein only a group-III element is supplied as a source material along with a carrier gas for formation of a Ga surface before a group-V element is supplied as a source material along with a carrier gas for the initial nitrization. FIG. 3 is a scanning electron microscopy photograph showing section and surface of a GaN layer formed over the GaAs substrate by a hydride vapor phase epitaxy. FIG. 2 is a fragmentary cross sectional elevation view illustrative of a GaN layer formed over a GaAs substrate. A GaAs substrate 101 has a surface tilted by 5 degrees from (100) surface toward [111]B. The GaAs substrate 101 is doped with CrO at a concentration of 0.33 weight ppm. GaCl is supplied along with a carrier gas onto this GaAs substrate 101 for formation of a Ga-rich surface 103 on the GaAs substrate 101 before a GaN low temperature growth buffer layer 105 is grown at a low temperature over the Ga-rich surface 103. Further, a GaN high temperature growth buffer layer 106 is grown at a high temperature over the GaN low temperature growth buffer layer 105. Both the GaN low temperature growth buffer layer 105 and the GaN high temperature growth buffer layer 106 are undoped. The concrete crystal growth processes are as follows.
While AsH.sub.3 is being supplied with a carrier gas of hydrogen, the substrate 101 is heated to a substrate temperature in the range of 630.degree. C. to 640.degree. C. for five minutes so that any oxide film is removed and an As surface is formed on the substrate 101. The substrate temperature is then dropped to 485.degree. C. GaCl is supplied together with a nitrogen carrier gas for thirty seconds to form a Ga-rich surface 103 on the As surface of the substrate 101. A GaN low temperature growth buffer layer 105 is then grown at a low temperature for thirty seconds on the Ga-rich surface 103. Finally, a GaN high temperature growth buffer layer 106 is grown at a high temperature of 700.degree. C. for thirty seconds on the GaN low temperature growth buffer layer 105.
FIG. 3 illustrates the of a rough surface of the GaN high temperature growth buffer layer 106. The planarity of the surface of the GaN high temperature growth buffer layer 106 is inferior. Such rough surface and inferior planarity of the GaN high temperature growth buffer layer 106 are caused by the Ga-rich layer 103 being not more than a Ga mono-atomic layer and an insufficient initial nitrization of the surface of the GaAs substrate 101 with supplying the group-V element along with the carrier gas.
As described above, if a GaN layer is grown over a GaAs substrate by the crystal growth, then the GaN layer is inferior in surface planarity.
In the above circumstances, it had been required to develop a novel method of crystal growth of a GaN layer with an extremely high surface planarity over a GaAs substrate.